Add transforms feature

This provides a mechanism for specifying conversions between SQL data
types and procedural languages.  As examples, there are transforms
for hstore and ltree for PL/Perl and PL/Python.

reviews by Pavel Stěhule and Andres Freund

contrib/hstore_plpython/hstore_plpython.c

@@ -0,0 +1,116 @@
+#include "postgres.h"
+#include "fmgr.h"
+#include "plpython.h"
+#include "plpy_typeio.h"
+#include "hstore.h"
+
+PG_MODULE_MAGIC;
+
+
+PG_FUNCTION_INFO_V1(hstore_to_plpython);
+Datum hstore_to_plpython(PG_FUNCTION_ARGS);
+
+Datum
+hstore_to_plpython(PG_FUNCTION_ARGS)
+{
+	HStore	   *in = PG_GETARG_HS(0);
+	int			i;
+	int			count = HS_COUNT(in);
+	char	   *base = STRPTR(in);
+	HEntry	   *entries = ARRPTR(in);
+	PyObject   *dict;
+
+	dict = PyDict_New();
+
+	for (i = 0; i < count; i++)
+	{
+		PyObject   *key;
+
+		key = PyString_FromStringAndSize(HS_KEY(entries, base, i), HS_KEYLEN(entries, i));
+		if (HS_VALISNULL(entries, i))
+			PyDict_SetItem(dict, key, Py_None);
+		else
+		{
+			PyObject *value;
+
+			value = PyString_FromStringAndSize(HS_VAL(entries, base,i), HS_VALLEN(entries, i));
+			PyDict_SetItem(dict, key, value);
+			Py_XDECREF(value);
+		}
+		Py_XDECREF(key);
+	}
+
+	return PointerGetDatum(dict);
+}
+
+
+PG_FUNCTION_INFO_V1(plpython_to_hstore);
+Datum plpython_to_hstore(PG_FUNCTION_ARGS);
+
+Datum
+plpython_to_hstore(PG_FUNCTION_ARGS)
+{
+	PyObject   *dict;
+	volatile PyObject *items_v = NULL;
+	int32		pcount;
+	HStore	   *out;
+
+	dict = (PyObject *) PG_GETARG_POINTER(0);
+	if (!PyMapping_Check(dict))
+		ereport(ERROR,
+				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+				 errmsg("not a Python mapping")));
+
+	pcount = PyMapping_Size(dict);
+	items_v = PyMapping_Items(dict);
+
+	PG_TRY();
+	{
+		int32		buflen;
+		int32		i;
+		Pairs	   *pairs;
+		PyObject   *items = (PyObject *) items_v;
+
+		pairs = palloc(pcount * sizeof(*pairs));
+
+		for (i = 0; i < pcount; i++)
+		{
+			PyObject *tuple;
+			PyObject *key;
+			PyObject *value;
+
+			tuple = PyList_GetItem(items, i);
+			key = PyTuple_GetItem(tuple, 0);
+			value = PyTuple_GetItem(tuple, 1);
+
+			pairs[i].key = PLyObject_AsString(key);
+			pairs[i].keylen = hstoreCheckKeyLen(strlen(pairs[i].key));
+			pairs[i].needfree = true;
+
+			if (value == Py_None)
+			{
+				pairs[i].val = NULL;
+				pairs[i].vallen = 0;
+				pairs[i].isnull = true;
+			}
+			else
+			{
+				pairs[i].val = PLyObject_AsString(value);
+				pairs[i].vallen = hstoreCheckValLen(strlen(pairs[i].val));
+				pairs[i].isnull = false;
+			}
+		}
+		Py_DECREF(items_v);
+
+		pcount = hstoreUniquePairs(pairs, pcount, &buflen);
+		out = hstorePairs(pairs, pcount, buflen);
+	}
+	PG_CATCH();
+	{
+		Py_DECREF(items_v);
+		PG_RE_THROW();
+	}
+	PG_END_TRY();
+
+	PG_RETURN_POINTER(out);
+}